This invention relates to methods and apparatus for fabricating polymeric articles, including single layer and multiple layer structures.
As used herein, "structures" means and includes single and multiple layer films, single and multiple layer sheets, and packages, including, by way of example, pouches, trays, lid stock, containers and the like made with those films and sheets.
The invention especially relates to the extrusion of a first polymeric composition which is sensitive to the thermal conditions to which it is exposed, at a first set of thermal conditions with a second polymeric composition which is preferably processed at a second set of more intense thermal conditions (e.g., higher temperature). A problem with respect to the type of extrusion to which this invention relates is that when the second set of conditions is sufficiently intense, the first polymeric composition can be adversely affected, e.g., changed or degraded, by use of conventional extrusion methods.
Vinylidene chloride copolymers (VDC) provide a barrier to transmission of moisture vapor and oxygen, and therefore are desirable for use when those properties are important. Since coextruding of a plurality of polymeric melt streams, in a desired relative structural relationship, at a single die exit is commonly practiced, it would be desirable to coextrude VDC with a wide variety of other polymeric compositions.
U.S. Pat. No. 3,558,330 Widiger et al relates to simultaneous extrusion of VDC with other polymeric materials. They include a table of suggested structures, at least one of which, namely MYLAR, cannot be "coextruded" because it is already in film form.
However, it is well known that VDC is sensitive to the thermal conditions to which it is exposed, see subsequent U.S. Pat. No. 4,379,117 to Baird Jr. et al. Baird Jr. et al suggest hollow cavities in the die, to be filled with circulating liquids at the temperature required to protect the VDC from overheating.
It is an object of this invention to provide other, and additional, means for reducing the impact of the higher temperature of the second polymeric melt stream element on the VDC, or other sensitive melt stream elements e.g., ethylene vinyl alchohol or nylon which are also temperature sensitive.
It is a more specific object to provide a method which comprises supercooling one or more of the melt stream elements.
Another specific object is to provide a method wherein the hotter melt stream element is joined with a melt stream element which is below the recrystallization temperature of the hotter melt stream element, and preferably extruding the joined melt stream before adverse change is manifested in the hotter melt stream element.
Still another specific object is to provide a heat sink as one of a plurality of melt stream elements.
Yet another specific object is to provide a modified die, and method of use, wherein the length of one of the paths in the die, in the direction of extrusion of the extrudate, is shorter than the length, in the same direction of the path traversed by another layer